Latency and energy aware value prediction for high-frequency processors

• Authors: Ravi Bhargava;Lizy K. John
• Venue: ICS '02 Proceedings of the 16th international conference on Supercomputing
• Year: 2002

This work addresses the issues of access latency and energy consumption in value predictor design for high-frequency, wide-issue microprocessors. Previous value prediction research allows for generous assumptions regarding table configurations and access conditions, while ignoring prediction latencies and energy issues. However, the latency of a high-performance value predictor cannot always be completely hidden by the early stages of the instruction pipeline as previously assumed, and it causes noticeable performance degradation versus unconstrained value prediction. This paper describes and compares several variations of basic value prediction methods: at fetch, post-decode, and decoupled.The performance of at-fetch and post-decode value predictors is limited by the high access latency of accurate predictor configurations. Decoupled value prediction excels at overcoming the high-frequency table access constraints by placing completion-time predictions into a separate and easily accessible storage. However, it has high energy requirements. We study a value prediction approach that combines the latency-friendly approach of decoupled value prediction with a more energy-efficient implementation. The traditional PC-indexed prediction tables are removed and replaced by a queue of prediction traces. This latency and energy aware method of maintaining and distributing speculated values leads to a 58%-95% reduction in value predictor energy consumption versus known value prediction techniques while still maintaining high performance.